The present invention relates to a DC-DC converter and more particularly, to a DC-DC converter controlled by a magnetic amplifier consisting essentially of a saturable reactor.
A magnetic amplifier-controlled, stabilized power supply comprising a saturable reactor has already been known. Since this type of power supply converts DC input voltage to stabilized DC output voltage, it is called a DC-DC converter. Of such power supplies, what is now widely used is a so-called Ramey-type, DC-DC converter which is of a forward-type having one switching element. One typical example of this Ramey-type DC-DC converter is shown in FIG. 1. This switching regulator (DC-DC converter) comprises a DC power supply 1 for supplying DC voltage to be stabilized, a transformer 2 having a primary winding 21 connected to the DC power supply 1 and a secondary winding 22, a transistor 3 whose collector is connected to the primary winding 21 and whose emitter is to the DC power supply 1. A self-oscillating circuit 4 in parallel with the DC power supply is connected to the base of the transistor 3 to turn on the transistor 3 on a periodical basis. Connected to the secondary winding 22 is a saturable reactor 5 to which an anode of a rectifying diode 6 is in turn connected. A cathode of the diode 6 is then connected to a choke coil 8 which leads to one output terminal 12. Another rectifying diode 7 is connected between the other end of the secondary winding 22 and the choke coil 8. A capacitor 9 is provided between output terminals 12, 13. The capacitor 9 and the choke coil 8 form a smoothing circuit, namely, an L-C filter. Terminals 12 and 13 are a positive output terminal and a negative output terminal, respectively. The other end of the secondary winding 22 of the transformer 2 is connected to the negative output terminal 13.
An emitter of a transistor 10 for controlling the resetting of the saturable reactor 5 is connected to the positive output terminal 12, and a collector thereof is connected to a point between the saturable reactor 5 and the anode of the diode 6. A base of the transistor 10 is connected to a reset control circuit 11. The reset control circuit 11 receives output voltage which is compared with reference voltage to produce an error signal. The error signal is applied to the base of the transistor 10 to turn on and off the transistor 10 so that proper reset current ir is supplied to the saturable reactor 5 for purpose of controlling the output voltage Vo. In this example, the output voltage Vo is used as a power source for reset current ir.
The operation of the above-mentioned switching regulator will be described. When the transistor 3 is turned on, voltage V.sub.1 is applied to the primary winding 21 of the transformer 2, and voltage V.sub.2 is induced in the secondary winding 22 which is positive at the dotted end thereof. It is to be noted that the saturable reactor 5 shows a high impedance until it is saturated. Because the saturable reactor 5 can be oppositely magnetized in advance, a period during which it shows a high impedance can be controlled. As a result, voltage is blocked thereby, with voltage V.sub.M, almost as high as V.sub.2, appearing between both ends of the winding of the saturable reactor 5.
After the saturation of the saturable reactor 5, the voltage V.sub.M decreases almost to zero, permitting the induced current to flow. As a result, voltage V.sub.2 ' appears between the anode and cathode of the diode 7. This is shown in FIG. 2.
When the transistor 3 is turned off, voltage of an opposite polarity is induced in the secondary winding 22. This voltage is usually called "flyback voltage." The flyback voltage will disappear during the OFF state of the transistor 3. To return the magnetization of the saturable reactor 5 to a desired original level, namely, to "reset" the saturable reactor 5, reset current ir is caused to flow through the saturable reactor 5 along the broken line shown in FIG. 1 by turning on the transistor 10. It is to be noted that the reset current ir flows through the secondary winding 22. In view of the direction of the reset current passing through the secondary winding 22, the transformer 2 retains a magnetic flux density which is higher than a residual magnetic flux density Br. This phenomenon is called "racheting" or "asymmetrical magnetization." The racheting reduces an operable range of the transformer 2, which means that the transformer 2 is easily saturated. This in turn may lead to the destruction of the switching transistor 3.
An article entitled "Saturation Phenomenon of Primary Transformer of Magnetic Control-Type Switching Regulator and Measures against It," Onda et al., Papers of Technical Meeting on Magnetics MAG-84-24, Jan. 28, 1984 discloses a magnetic control-type switching regulator whose circuit is shown in FIG. 3. It is to be noted that this switching regulator, also often called a DC-DC converter, has essentially the same structure as that of FIG. 1, except that in the switching regulator of FIG. 3, a saturable reactor 5 is connected to a negative end of a secondary winding 22, and a path for reset current ir is formed along a transistor 10, a diode 15 and a saturable reactor 5 and back to a negative output terminal 13. Incidentally, the negative end of the secondary winding 22 means that such end of the secondary winding 22 is negative while the transistor 3 is in a turn-on state. As for other elements, the same reference numerals are assigned in FIGS. 1 and 3.
It is clear that while the transistor 3 is in a turn-off state, the reset current ir does not flow through the secondary winding 22. Thus, the racheting of the transformer 2 can be avoided. Nevertheless, this switching regulator has a problem that the saturable reactor 5 undergoes extraordinary temperature rise during the operation. The cause of such extraordinary temperature rise is that resonance current is generated during the turn-off period of the transistor 3 by junction capacitance of the switching transistor 3, capacitance in a snubber circuit connected in parallel to the transistor 3 for absorbing surge voltage which appears at the time of turning off the transistor 3, and inductance of the primary winding 21 of the transformer 2, and that it is applied to the saturable reactor 5 through magnetic coupling of the primary and secondary windings 21 and 22.
This phenomenon will be explained in detail by means of FIG. 4 which is an equivalent circuit to FIG. 3. It is to be noted that the transformer 2 is expressed by an exciting inductance Lex and two leakage inductances Ln.sub.1 and Ln.sub.2, and that a capacitor 50 is inserted in parallel with the switching transistor 3. The capacitor 50 represents part of a snubber circuit. During the turn-off period of the transistor 3, discharge current id, which may be called "resonance current," flows from the capacitor 50, inducing voltage V.sub.L in the exciting inductance Lex. The relation between the discharge current id and the induced voltage V.sub.L is schematically shown in FIG. 5. The discharge current id begins flowing when the transformer 2 has been reset, namely when the transformer's core has returned to an original state in which it has a residual magnetic flux density Br. As is clearly shown in FIG. 5, while the discharge current id is decreasing, the induced voltage V.sub.L is positive and increasing. This voltage V.sub.L is applied to the saturable reactor 5. A positive part of the voltage V.sub.L is shown by a hatched portion in FIG. 2. Since this resonance current causes the magnetization of the saturable reactor 5, it contributes to the temperature rise of the saturable reactor 5.
The above-mentioned article entitled "Saturation Phenomenon of Primary Transformer of Magnetic Control-Type Switching Regulator and Measures against It" also discloses a magnetic control-type switching regulator equipped with a saturable reactor 5 having first and second windings, which is shown in FIG. 6. An essential feature of this switching regulator is that a saturable reactor 5 has two windings; a first winding 52 connected to an end of a secondary winding 22 which is negative while a switching transistor 3 is in a turn-on state, and a second winding 53 connected between a transistor 10 connected to a positive side of a capacitor 9 and a diode 15 which is in turn connected to a negative output terminal 13. Reference Numeral 51 denotes a magnetic core of the saturable reactor. In this switching regulator, too, reset current ir flows along the broken line shown in FIG. 6, without passing through the secondary winding 22 of the transformer 2.
FIG. 7 shows an equivalent circuit to FIG. 6. It is to be noted that since the first and second windings 52 and 53 of the saturable reactor 5, respectively, are magnetically coupled with each other, the reset current ir flowing through the second winding 53 induces current iM in the first winding 52. Since these two windings 52 and 53 have the same polarity, namely their positive polarity ends are on the same side, the current iM flows through Lex in the same direction as the discharge current id. Therefore, while the discharge current id is decreasing, the current iM substantially offsets the decrease in the discharge current id, leading to minimal voltage V.sub.L. If the saturable reactor 5 has such a turn ratio that the induced current iM may sufficiently offset the decrease in the discharge current id, the transformer 2 would generate substantially no voltage despite the discharge current. This is schematically shown in FIG. 8. Because of this feature, the saturable reactor 5 is free from extraordinary heating.
Despite the above advantages, the switching regulator of FIG. 6 still has a problem, which is detrimental to switching regulators as means for supplying stabilized DC voltage. The problem is that it generates noises which are not always sufficiently low. Particularly in view of the recent trend that regulations, such as those of U.S. Federal Communication Committee (FCC) and Verband Deutscher Elektrotechniker (VDE) are becoming stricter on switching regulation's noises, reducing a noise level as low as possible is extremely desired.